Magnetic resonance imaging (MRI) may be used to form images of the brain's anatomy and the physiological processes of the brain. MRI brain scans can use a strong, permanent and static magnetic field to align nuclei in the brain region being studied. Another magnetic field, the gradient field, is then applied to spatially locate different nuclei. Finally, a radio frequency (RF) pulse is generated to kick the nuclei to higher magnetization levels, with the resulting effect depending on where they are located. When the RF field is removed, the nuclei go back to their original states, and the energy the nuclei emit is measured with a coil to recreate the positions of the nuclei. MRI thus provides a static imaged view of the brain tissue.
Functional magnetic resonance imaging or functional MRI (fMRI) is a functional brain imaging procedure using MRI (magnetic resonance imaging) technology that measures brain activity by detecting changes associated with blood flow. This technique relies on the fact that cerebral blood flow and neuronal activation are coupled. When an area of the brain is in use, blood flow to that region tends to increase.
fMRI can use the properties of oxygen-rich blood in imaging. The central thrust behind fMRI may extend MRI to capture functional changes in the brain caused by neuronal activity. Differences in magnetic properties between arterial (oxygen-rich) and venous (oxygen-poor) blood can provide this information because changes in blood flow and blood oxygenation in the brain may be linked to neural activity.
One form of fMRI uses the blood-oxygen-level-dependent (BOLD) contrast method. This is a type of specialized brain scan used to map neural activity in the brain by imaging the change in blood flow (hemodynamic response) related to energy use by brain cells. In recent decades, fMRI has come to dominate brain mapping technology because fMRI does not use invasive medical procedures on patients (e.g., shots, surgery, ingested substances, or exposure to radiation, etc.). Other methods of obtaining contrast are arterial spin labeling and diffusion MRI. The latter procedure is similar to MRI but uses the change in magnetization between oxygen-rich and oxygen-poor blood as its basic measure. This measure is frequently corrupted by noise from various sources and hence statistical procedures are used to extract the underlying signal. The resulting brain activation can be presented graphically by coding the strength of activation across the brain or the region studied. The technique can localize activity to within millimeters but, typically, no better than within a window of a few seconds.